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SubjectRe: Errors in the VM - detailed (or is it Tux? or rmap? or those together...)
Date
On Saturday den 2 February 2002 16.38, Roy Sigurd Karlsbakk wrote:
> > I have reread the first mail in this series - I would say that Bug#2 is
> > much worse than Bug#1. This since Bug#1 is "only" a performance problem,
> > but Bug#2 is about correctness...
> >
> > Are you 100% sure that tux works with rmap?
>
> Of course not. How can I be sure???
>
> > I would suggest testing the simplest possible case.
> > * Standard kernel
> > * concurrent dd:s
>
> Won't work. Then all I get is (ref prob #1) good throughput until RAMx2
> bytes is read from disk. Then it all falls down to ~1MB/s. See
> http://karlsbakk.net/dev/kernel/vm-fsckup.txt for more details.

How do you know that it gets into this at RAMx2? Have you added 'bi' from
vmstat?

One interesting thing to notice from vmstat is...

r b w swpd free buff cache si so bi bo in cs us sy id
When performing nicely:
0 200 1 1676 3200 3012 786004 0 292 42034 298 791 745 4 29 67
0 200 1 1676 3308 3136 785760 0 0 44304 0 748 758 3 15 82
0 200 1 1676 3296 3232 785676 0 0 44236 0 756 710 2 23 75
Later when being slow:
0 200 0 3468 3316 4060 784668 0 0 1018 0 613 631 1 2 97
0 200 0 3468 3292 4060 784688 0 0 1034 0 617 638 0 3 97
0 200 0 3468 3200 4068 784772 0 0 1066 6 694 727 2 4 94

No swap activity (si + so == 0), mostly idle (id > 90).
So it is waiting - on what??? timer? disk?

>
> > What can your problem be:
> > * something to do with the VM - but the problem is in several different
> > VMs... * something to do with read ahead? you got some patch suggestions
> > - please use them on a standard kernel, not rmap (for now...)
>
> Then fix the problem rmap11c fixed. I first need that fixed before being
> able to do any further testing!

Roy, did you notice the mail from Andrew Morton:
> heh. Yep, Roger finally nailed it, I think.
>
> Roy says the bug was fixed in rmap11c. Changelog says:
>
>
> rmap 11c:
> ...
> - elevator improvement (Andrew Morton)
>
> Which includes:
>
> - queue_nr_requests = 64;
> - if (total_ram > MB(32))
> - queue_nr_requests = 128;
> + queue_nr_requests = (total_ram >> 9) &
> ~15; /* One per half-megabyte */
> + if (queue_nr_requests < 32)
> + queue_nr_requests = 32;
> + if (queue_nr_requests > 1024)
> + queue_nr_requests = 1024;

rmap11c changed the queue_nr_requests, that problem went away.
But another one showed its ugly head...

Could you please try this part of rmap11c only? Or the very simple one
setting queue_nr_request to = 2048 for a test drive...

/RogerL

--
Roger Larsson
Skellefteå
Sweden
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